In recent vehicles a passenger protection system such as an airbag or a seat belt pretensioner is used. In case of the airbag system, an inflator is activated to inflate an airbag when an acceleration sensor detects a vehicle collision. Since the voltage of a storage battery of the vehicle is not sufficient to activate the inflator by its associated circuits and devices, a voltage booster circuit is used to supply a higher voltage to the circuits and devices. The voltage booster circuit is typically a DC-DC converter, which boosts a battery voltage, and has a back-up circuit to supply sufficient electric power to activate the inflator even when the electric connection with the battery is disconnected due to a collision.
JP 2001-178118A discloses one exemplary airbag system. In this system, a DC-DC converter circuit has a switching transistor, a booster coil and the like. The switching transistor is turned on and off at a fixed frequency, specifically with a fixed on-period and a fixed off-period, to boost a battery voltage and store the boosted voltage in a capacitor of the back-up circuit. This DC-DC converter circuit, however, needs a long time to charge the capacitor. Further, the fixed frequency to turn on and off the switching transistor is set to about several tens of kHz to several hundreds of kHz to ensure switching operation of the switching transistor even under a low battery voltage condition. This switching operation of the transistor at the high frequency tends to generate noises.